Plant different varieties of tomato to get a longer harvest. Photo / The New Zealand Listener

We have bad sooty mould on a lemon tree and an orange tree. We have continuously sprayed the trees with washing water and have opened up the lemon tree so that it is well ventilated but the mould persists.

- Lin Drake

Mix a tablespoon of baking soda, a tablespoon of dish soap and a tablespoon of vegetable or neem oil ( I mostly use olive oil) into a litre of water. The baking soda will deal to the mould and the oil will kill the insects that are causing the problem. Don't forget to spray on the underside of the foliage.

Our apple tree has leaf roller. My husband has been picking off the affected leaves but he wondered if there was something we could use to spray it. We still have a little bit of blossom and loads of fruit have already set. It's espaliered against the fence so it is easy to get at. We were thinking of using neem oil but wondered if that would be adequate. Do you have any other suggestions please.

- Many thanks, Marie

The leaf roller caterpillar is the main culprit here. It basically eats one side of a leaf causing it to roll up which is very cunning because this makes it hard to eradicate. Organic sprays are available which contain naturally occurring soil bacteria like Bacillus thuringiensis (Kiwi Care do one called Organic Caterpillar Bio Control); this is safe on food and easy to use. For a long-term solutions make sure you keep weeds and grasses away from your trees as this can provide a habitat for the caterpillars. Bio controls include ladybirds which can be encouraged into your garden by using organic pest control methods and paper wasps, which you might not wish to encourage. But the digital method (literally picking them off with your fingers) is also useful.

The vegetable garden has oxalis in it. Is there any solution other than spraying and sacrificing a crop for a year?

- Lin Drake

Try no-dig gardening. Place thick cardboard over the offending ground then go over this with 100mm of compost and 100mm of top soil. Plant straight into that and repeat annually. Basically you'll bury the oxalis bulbs.

Please advise the best type of grafted tomatoes to grow in Auckland for medium-size fruit with flavour. What is best to feed them with, liquid or dry fertiliser?

- Regards, Sid Irving

I think that the solid all-round performers are Moneymaker, Potentate and Grosse Lisse. You could also try Black Kim or the purple Russian varieties To get a longer supply of fruit, plant several varieties. Moneymaker is an early cropper, while Black Kim and most of the Russian varieties are late, so maybe mix it up a bit. Dress the holes with milk powder and water or spray the plants with milk spray (60 per cent water 40 per cent milk powder) once every 10 days to ward off black spot. Stake the plants as soon as you plant them to avoid disturbing them at a later date.

We live in Pukekohe and have very good soil, growing most of our vegetables. The one problem we have been unable to find a satisfactory answer for is growing carrots that are not affected or holed by carrot fly. Have you any suggestions?

- Thanks, Marie

Carrot fly becomes a problem from around October when the larvae hatch and begin boring into new carrots. I don't think it's a problem particular to Pukekohe (and I'm dead jealous of your amazing soil by the way). You need to break the cycle of the fly so I recommend covering the crop with micro cloth which is very fine and will keep out almost all insects.

Utilising crop rotation is important also, and avoid planting root crops such as parsnip nearby as well. These are great host plants for larvae over winter, which you don't need. Lastly keep the beds immaculately clean and weed free. This always goes a long way to reducing pests.

Spray rhododendrons to combat insect pests.

TO DO THIS FORTNIGHT

Vege gardenDry beds equal stressed plants, so watering at this time of the year is all-important. Irrigate to maintain soil moisture rather than watering when the beds have dried out. The addition of pea straw as mulch can also be advantageous in most situations.

Try to keep the weeds at bay especially around garlic and onion crops. Direct sow salad greens such as lettuce and rocket and herbs such as coriander and basil.

Plant kumara shoots in a free-draining, well worked-over soil. To keep the runners tidy I grow them up stakes which have the added bonus of stopping the plant developing lots of small tubers rather than nice big fat ones.

If your peas are looking a bit tired and the mildew has taken hold, pick the remaining pods, then remove and compost the stalks.

It's not too late to plant cucurbits such as cucumber, pumpkin and zucchini. You may even get away with watermelon. Make sure you give these plants a good start with plenty of sun, water and compost or sheep pellets dug into a rich free-draining soil. Plant cucurbits on mounds in heavy soils. Cucurbits are also excellent companions for corn. Crops like pumpkin or cucumber grow along the ground and protect the corn roots from too much sun.

Harvest comfrey and soak it in a barrel along with seaweed. Over a period of six weeks or so it will become an excellent tea or spray for late summer crops.

If you're late to get seeds up and growing then get punnets from the garden centre.

It's a good time to plant leeks and celery. These won't be fully mature until winter but celery can be treated like a herb and can easily have the odd branch picked, while young leeks are a perfect accompaniment to any late summer barbecue. Another winter crop to get in the ground now is parsnip.

FlowersKeep hydrangeas well watered and mulched. You can help maintain the colour by altering the trace elements in the soil. Lime will help the flowers keep their red colour and aluminium sulphate you are able to keep the blues looking blue.

For some strong festive colour try New Guinea impatiens or phlox. Both have strong pastel colours and do well in semi-shade. These two are the perfect foil for some of the harder plant forms found in a subtropical garden. Dry time favourites include knopfia (red hot poker), grevillea and hebe. Mulch and stake dahlias before they get too established.

Feed gardenias with magnesium sulphate to help ward of yellowing leave. Make sure they are well watered and dress with compost to cover the delicate shallow roots. Mulch and feed rhododendrons. These have just finished flowering and are piling on new growth. Keep well watered to fight infestations of sucking insects like thrips.

Landscape and lawnsKeep lawns well watered and mow them long (5cm). If the grass is yellowing, increase the watering programme slightly and dress with some fine compost and lime. If the grass is still yellow then fertilise with a lawn fertiliser. Take care not to over-fertilise though as this can promote soft growth which is easily damaged by foot traffic and becomes greasy. I use a mixture of urine (my own of course) water and chicken poo which does the trick.

Spray your citrus, viburnums, rhododendrons and the like prophylactically with oil and baking soda spray (one tablespoon each of vegetable/olive oil, garden friendly dish soap and baking soda in 1 litre of water). Spray the undersides of the leaves once a week to combat white fly, psyllids and thrips.

Feed hedges with compost and trim lightly if necessary. It's important to remove all of the plant material to prevent fungal infections.

Do you have a gardening, DIY or landscaping question for Justin? Email gardening@nzherald.co.nz with your question and we will be happy to answer it in our pages.